Abstract
Emergence of methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant coagulase-negative staphylococci (MR-CoNS) in bovine milk is a major public health concern. The primary purpose of this research was to determine molecular genetic characteristics and antibiotic resistance of staphylococcal isolates recovered from milk of mastitic cows in the Shaanxi Province in Northwestern China. One hundred and thirteen methicillin-susceptible Staphylococcus aureus (MSSA), one mecA-positive and phenotype-positive MRSA, seven mecA- and mecC- negative but phenotype-positive MRSA and two MR-CoNS including one oxacillin-susceptible mecA-positive Staphylococcus haemolyticus (OS-MRSH) and one mecA-positive and methicillin-resistant Staphylococcus epidermidis (MRSE) isolates were recovered from 214 quarter milk samples on 4 dairy farms. All above 123 isolates were subjected to antibiotic resistance profiling. S. aureus isolates were also genotyped using the spa typing and the multilocus sequence typing (MLST). Eight MRSA and 2 MR-CoNS isolates were additionally tested for SCCmec types. Resistance was common among isolates against ampicillin or penicillin (80.5%), kanamycin (68.3%), gentamicin (67.5%), tetracycline (43.9%) and chloramphenicol (30.1%). However, no isolate was resistant to vancomycin or teicoplanin. Twenty, 29 and 58 isolates showed resistance to 1, 2 or more than 2 antibiotics, respectively. The predominant multidrug resistance profile was penicillin/ampicillin/kanamycin/gentamicin/tetracycline (46 isolates). Most S. aureus isolates belonged to spa types t524 (n = 63), t11772 (a new type, n = 31) and t4207 (n = 15). At the same time, MLST types ST71 (n = 67) and ST2738 (a new type, n = 45) were identified as dominant sequence types. The mecA-positive and phenotype-positive MRSA isolate had a composite genotype t524-ST71-SCCmecIVa, while 7 mecA-negative but phenotype-positive MRSA isolates were all t524-ST71. The OS-MRSH isolate contained a type V SCCmec cassette, while the MRSE isolate possessed a non-typeable SCCmec. The spa-MLST types t11772-ST2738 (n = 27), t11807-ST2683 (n = 4) and t11771-ST2738 (n = 3) were newly identified genotypes of S. aureus. These new genotypes and multidrug-resistant staphylococci could pose additional threat to animal and human health.
Highlights
Milk and milk products are common vehicles of staphylococcal food poisoning [1]
Milk samples were obtained from dairy cows with naturally occurring clinical mastitis under the ethical approval granted by the College of Animal Science and Technology, the Northwest Agriculture and Forestry (A&F) University (Permit Number: NWAFU1008)
One hundred and twenty one S. aureus and 336 CoNS including 2 methicillin-resistant coagulasenegative staphylococci (MR-CoNS) isolates were recovered from 214 milk samples
Summary
Appearance of methicillin-resistant S. aureus (MRSA) in milk and dairy foods has become a major concern for animal and human health. While both MRSA and MSSA can acquire many antimicrobial determinants, MRSA are resistant to practically all β-lactam antibiotics [2]. Previous studies have reported appearance of MRSA in bovine mastitic milk in several countries, including Switzerland [5], Korea [6], Belgium [7] and Turkey [8]. There have been a few reports on MRSA from bovine milk in China. In addition to MRSA, methicillin-resistant coagulasenegative staphylococci (MR-CoNS) from mastitic milk have been reported in various countries [11, 12]. There is no report in the literature about MR-CoNS from milk in China
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